Shelf



FIG. 1 is a reduced perspective view of a shelf showing our new design;

FIG. 2 is a top plan view thereof;

FIG. 3 is a front elevational view thereof;

FIG. 4 is a side elevational view thereof;

FIG. 5 is a rear elevational view thereof;

FIG. 6 is a bottom plan view thereof. 

The ornamental design for a shelf, as shown and described. 